The quantity of municipal waste generated each day creates numerous problems and costs to society. Among these are the huge volume of landfill space consumed each day, the fuel costs and emissions involved in transporting these wastes to the landfill, the ecological liabilities in both air and water emissions associated with burying these wastes, the lack of any energy benefit from most of the green house gas emissions coming from the landfill, the leaching of materials into the waste water coming from the landfills, and the tremendous loss of valuable energy and material resources buried within the landfill.
Some regions and municipalities have created Municipal Recycling Facilities (MRFs), which capture a portion of the material resources from the waste stream, such as glass, metals, plastics, and paper. Others capture little if any of these materials, and in any case, there are very significant material resources that still end up in the landfill solid waste stream.
The application of waste-to-energy (WTE) captures the bulk of the recoverable energy from such waste, and, in the process, reduces the solid waste going to the landfill by around 70-80%; conserving landfill space, reducing landfill-related emissions, and reducing the transportation fuel consumed and its associated cost and emissions. WTE ash generally consists of two types or components of ash, designated as bottom ash and fly ash. The relative portions of bottom ash vs. fly ash depend on the manner of preparation of the municipal solid waste prior to burning and on the manner of burning. By way of example only, bottom ash commonly will constitute around 70-80% of the total WTE ash and fly ash will be around 20-30%. The “bottom ash”, i.e. the larger portion of the ash that remains on the “bottom” of the incinerator hearth after combustion, contains the bulk of recoverable metals. Fly ash is made up of the smallest, least massive particles of ash that are carried upward with the exhaust gases produced during incineration. The concentration of heavy metals in fly ash can and often does, make them a hazardous waste because it does not pass leaching toxicity testing (TCLP, Toxicity Characteristic Leaching Procedure).
Some WTE facilities do an additional post-combustion processing of the bottom ash to further remove ferrous and nonferrous metals from the ash prior to disposal. The conventional approach for ash recycling utilizes single-stage magnetic separation to remove iron and steel followed by an eddy current separator to remove some of the remaining nonferrous metals (see, e.g., FIG. 1A). Eddy current separators conventionally could be used to separate metal particles only down to around 0.25 inches, while more modern higher-frequency eddy current separators can separate down to around 0.125 inches but only at a large capital cost per ton of capacity. Therefore, due to limitations of the conventional technologies, current ash recycling approaches miss a substantial amount of metal, particularly the smaller metal particles and non-ferrous materials.
Furthermore, the metal concentrates produced by WTE facilities have relatively low metal concentrations, typically in the range of about 50-70% metal. More specifically, because substantial amounts of remnant ash remain associated with these metal concentrates, the resultant metal concentration (i.e. the quality and value of the metal concentrates) is low. There is also a relatively low level of liberation of the metal from the ash, which has the effect of reducing the percentage of recovery of the metals within the ash stream, particularly for the smaller metal sizes.
Ash has the commercial potential for use in construction material applications (e.g. an aggregate in concrete). In some instances, the chemical composition of the ash can also make it useful as a chemical raw material additive to a process. However, the applicability of WTE ash for use as a product is related to how “clean” the ash is with regard to its chemical composition and the completeness of metal removal. Remnant metals in the ash typically cause a more reactive ash product in which the remnant metal particles can corrode and react with the environment, and leach metals during its life cycle. Such leaching presents potential environmental liabilities and product life/performance liabilities for product applications using the ash. Therefore, when a significant amount of magnetic and nonferrous metals and materials are left behind in the ash, the applicability and appropriateness of such ash for use as a product rather than as a landfill waste is low.
There remains, therefore, a need for an ash recycling method and system for removing and recovering free and combined metals from combustion ash that 1) increases the yield of the recovered metal products and improves the quality of the recovered metal products; and 2) where appropriate, increases the yield of a cleaner ash and improves the properties of the clean ash to produce a superior, higher quality mineral sand product that can be utilized in concrete and other construction materials and other product applications.
The process described herein achieves a high level of liberation of metals and other phases within the ash to enhance the separation and extraction of high quality metal concentrates, and the process technologies for extracting the metal concentrates operate to a fine particle sizes. The amount, size and shape of the metal particles, the quantity of metal particles trapped within or tightly adhered to the ash particles, and likewise the quantity of ash particles trapped within or tightly adhered to metals within the ash, vary widely from ash-to-ash depending upon the composition of the combustion ash and the technology used to create the ash. As described herein, substantial liberation of metal particles from the ash and the use of processes to facilitate extraction and concentration of smaller particles improves metal recovery.
Furthermore, in certain embodiments (by way of example only, when bottom ash is used as an ash source), the process can produce a “clean” ash that is commercially valuable for end-product applications. More specifically, the ash produced has an extremely low remnant metal content which enhances the quality of the ash and its product applicability. The near-absence of any remnant metals in the ash makes the ash less reactive, more stable, less susceptible to corrosive reactions and the associated deleterious effects of corrosion on ash products, and a superior recycled raw material and aggregate for use in ash-based products. The resulting clean ash product is more appropriately referred to as a “mineral sand” or “mineral aggregate”.
Thus, the current invention can produce superior quality in the metal end-products, and at times the ash product, plus superior economic value for those products.